The thermal printer is a device for carrying out a print operation by driving multiple heating elements that constitute a thermal head in the form of a line. A maximum number of dots that can be driven simultaneously among all the heating elements arranged in the form of a line are subjected to a time-sharing drive.
A reason why such time-sharing drive is employed is as the following; if all the heating elements are driven simultaneously, power consumption is increased and the voltage applied on each of the heating elements is lowered. Lowering of the voltage that is applied on each of the heating elements may cause a deterioration of print density and uneven print quality.
In view of the problem above, the maximum number of dots that can be driven simultaneously is preset, and the heating elements arranged in one line is segmented and driven in units of some heating elements, the number of which corresponds to the maximum number of dots being preset as described above. By way of example, if the maximum number of dots that can be simultaneously driven is preset as 64 dots among the thermal head in which 256 dots of heating elements are arranged in one line, the one line is divided by four (4=256/64), and four times of driving are performed using 64 dots as a unit, so as to drive all of the dots within one line.
FIG. 18 illustrates the drive of the thermal head using the segmented blocks. Here, the thermal head 200 is made up of the heating elements 201 being connected with one another, the total number of which is N. If it is assumed that the number of the heating elements that is allowed to be energized simultaneously is n, under to the constraint of power supply capacity, the heating elements 201, the total number of which is N, are segmented into the blocks, each including n heating elements 201, according to the relationship between the total number N and the simultaneous-energization possible number n, and then, power feeding is performed for each of the segmented blocks. FIG. 18 illustrates the case where the number of the segmented blocks is assumed as eight.
FIG. 18A illustrates a drive state when the total number of dots to be energized in one line is less than the simultaneous-energization possible number n. If the number of dots to be energized is small, it is possible to energize one line at one time, thereby shortening a print cycle and raising a print speed. FIG. 18B illustrates a drive state when the total number of dots to be energized in one line is more than the simultaneous-energization possible number n. If the number of dots to be energized is large, it is not possible to energize one line at one time due to the constraint of the power supply capacity, and therefore, the energization is performed for each of the segmented blocks. Accordingly, the print cycle becomes longer and the print speed is lowered.
A larger maximum number of dots possible for the simultaneous drive may achieve a higher print speed. However, as described above, if the number of dots of the heating elements that are simultaneously driven is increased, the voltage drop may be enlarged by that much, an output voltage of the power supply becomes equal to or lower than a voltage level that guarantees proper operation, and a proper print operation is not guaranteed.
The voltage drop depends on inner electrical resistance of the power supply, resistance of the head, resistance of the other parts, and the like, and those resistance values are variable depending on production tolerance and electrical property. Therefore, conventionally, the factors above are considered, and the maximum number of dots possible for the simultaneous drive is preset assuming that the voltage of the power outlet terminal is under the worst condition being anticipated.
The heating elements within one line are segmented into blocks and energization is performed in units of the segmented block, whereby it is possible to resolve the constraints of power supply capacity. However, there is a problem that the configuration above may result in proportionately lowered print speed. As a method for resolving such lowering of the print speed, it is known that the cycle is set to be variable according to the number of segmented blocks.
However, it has been pointed out that if the speed is set to be variable, a printed dot length is also made variable, thereby causing another problem that a difference occurs in the length of printing.
FIG. 19A and FIG. 19B illustrate fluctuations of the print length, due to the variable print speed. In FIG. 19B, the dot length is represented by the product (v·t) of a speed v for transporting a print sheet and a pulse width t for feeding power into the heating element. A difference in the transport speed may cause a difference between the dot length Lf (=vf·t) when the print sheet is transported at a high transport speed vf, and the dot length Ls (vs·t) when the print sheet is transported at a low transport speed vs. As shown in FIG. 19B, this difference in the dot length L appears in the form of gap d between the lines.
In order to solve the problem above, there is suggested a drive method in which the print speed is made variable according to the division number when segmented into blocks, as well as the energization pulse width for energizing the heating element is made variable according to the print speed (see Patent document 1).
FIG. 19C and FIG. 19D illustrate the drive method in which the energization pulse width for energizing the heating element is made variable according to the print speed. Here, the energization pulse width is assumed as t when the print speed is high, and when the print speed is low, the energization pulse width is assumed as t′, which is set to be longer than t. By setting the energization pulse width to be variable, the dot length Ls in the case of the low transport speed vs is adjusted to vs·t′, which agrees with the dot length Lf in the case of the high transport speed vf, thereby resolving the difference in dot length L that is caused by the speed difference.    Patent document 1: Japanese Examined Patent Application Publication No. 8-25291    Patent document 2: Japanese Unexamined Patent Application Publication No. 6-191080